The present invention relates to an optical beam scanning device which is used for laser printers and digital copying machines.
The optical beam scanning device comprises a pre-deflection optical system, an optical beam scanning device and an imaging optical system.
The pre-deflection optical system has a polarity of lenses for directing a light beam emitted from a light source device (e.g., laser diode) to the optical beam scanning device and an opening portion with a predetermined shape of opening. The pre-deflection optical system adjusts the cross-sectional configuration of the light beam from the light source device to a predetermined one and guides the resultant beam toward a predetermined position on the optical beam scanning device.
The optical beam scanning device has a polygon mirror with a plurality of reflecting surfaces (mirror surfaces). The optical beam scanning device successively deflects the light beam from the pre-deflection optical system with its configuration having been adjusted to the predetermined one while continuously rotating its reflecting surfaces in order to generate a light beam which is to be guided from one end to the other end of an image surface (so-called photosensitive body) in its width direction along its rotating direction (main scanning direction). Accordingly, light beams illuminated onto the reflecting surfaces of the polygon mirror have their reflecting angles changed successively by the reflecting surfaces of the polygon mirror being rotated and then reflected (deflected) in the main scanning direction.
The imaging optical system scans substantially linearly and images, in a direction along its axial direction, the light beam successively reflected by the reflecting surfaces of the polygon mirror on a predetermined position of the image.
An imaging lens (generally referred to as an fθ lens) is used for the imaging optical system. The imaging lens converges the light beams reflected by the reflecting bodies of the polygon mirror onto a focal position on the image surface and makes diameters of the light beams, reflected by the reflecting surfaces of the polygon mirror, uniform.
Further, the imaging optical system controls so that the speed of light beam which is reflected from the reflecting surfaces of the polygon mirror and which is to be scanned on the image surface is constant over the entire scanning area.
Patent Reference 1 (Japanese Patent Application Laid-Open (JP-A) No. 8-171069) and Patent Reference 2 (JP-A No. 11-149055) disclose conventional optical beam scanning devices.
Patent Reference 1 discloses the arrangement in which a first imaging lens with negative power is provided next to a polygon mirror in an imaging optical system for an optical beam scanning device, and positive power at a scanning end position finally becomes larger than that at a scanning central position by combination of a plurality of imaging lenses.
Patent Reference 2 discloses the arrangement formed of a single imaging lens with positive power.
In general, the scanning position on the image surface in the main scanning direction is substantially in the linear relationship with a scanning angle determined by the reflecting surfaces of the polygon mirror. Accordingly, as the scanning angle is increased, the scanning position is shifted from the central position.
If the imaging lens is not placed in the imaging optical system, as the scanning angle is increased, the scanning position is shifted greatly from its target position (ideal position). Conventional devices do not compensate for this shift perfectly.
According to conventional examples, the main scanning direction diameter of beam scanned on the image surface varies depending on its scanning position.
An overfilled type optical beam scanning device solves the aforementioned drawbacks by increasing the number of reflecting surfaces for a polygon mirror and decreasing an available scanning angle. Nevertheless, when a scanning area with a certain size is to be obtained, an optical path length is increased. This results in a large scale of structure and an increased number of folded mirrors mounted in an optical beam scanning device. As a result, costs are increased.
The above-described Patent References 1 and 2 disclose techniques for solving such problems. Nevertheless, the optical beam scanning device of Patent Reference 1 has a plurality of imaging lenses and thus the problem about an increased number of lenses may arise.
The optical beam scanning device of Patent Reference 2 includes a single imaging lens. Nevertheless, the scanning angle is increased and the time for non-emission area from the end position of a scanning line to the start position of the next scanning line in the main scanning direction is reduced. As a result, it is difficult to ensure control time for APC (Auto Power Control) in a light source device.